The invention relates to a method of manufacturing ceramic sintered bodies. In the method the starting material includes the basic material of the sintered body to be manufactured and with the addition of compounds which form a melting phase with the basic material but which are not incorporated in the crystal lattice of the sintered body.
Alkaline earth titanates, alkaline earth zirconates, alkaline earth stannates or mixed crystals thereof are, for example, the basic material of the sintered body for a series of important electronic components. Such components are for example, ceramic multilayer capacitors, grain boundary barrier layer capacitors, or PTC resistors. Both in these capacitors and in these resistors, the electrical properties are determined principally by the ceramic microstructure.
In such a ceramic capacitor, for example, the value of the dielectric constant of the material and the temperature dependence thereof vary with the average grain size of the material. In PTC resistors, for example, the value of the resistance step and of the cold resistance vary with the average grain size and the homogeneity of the ceramic structure. In boundary grain barrier layer capacitors the breakdown strength and the insulation resistance, vary with the grain size and the grain size distribution. Methods of controlling and influencing the microstructure of the corresponding ceramic sintered bodies are hence of great importance in the manufacture of the these electronic components.
The ceramic microstructure, and, hence the average grain size snd the homogeneity of the ceramic structure, is also an important parameter in other ceramic sintered bodies used in electronic components. For example, in an oxide ceramic which is used in high temperature processes the lifetime, the strength and the temperature fluctation resistance are also determined by the ceramic microstructure. The same applies, for example, to oxide ceramics for which particularly good mechanical properties are required, for example for cutting tools and for machine construction.
Experiments on the grain growth of barium titanate as a basic starting material for a ceramic sintered body have demonstrated that samples, with a small excess of TiO.sub.2 or other additions which form a melting phase with the barium titanate at a given temperature show discontinuous grain growth (hence a recrystallization) above the eutectic temperature. This phenomenon applies equally to other starting materials with additions forming a melting phase.
For example, in barium titanate with an excess of TiO.sub.2, Ba.sub.6 Ti.sub.17 O.sub.40 forms a melting phase with BaTiO.sub.3 at temperatures of 1310.degree. to 1320.degree. C. Discontinuous grain growth (recrystallization) in the barium titanate and its mixed crystals is also found at temperatures around 1050.degree. C. to 1200.degree. C. by other additions, for example, mixtures of CuO--Cu.sub.2 O with a further metal oxide. Experiments performed by applicants have demonstrated that additions of CuO.MeO.sub.x, in which MeO.sub.x is the oxide of at least one element of group III, V, VI or VII of the periodic table of elements, as a compound forming a melting phase lead to an increased grain growth in alkaline earth titanates, alkaline earth zirconates, alkaline earth stannates and mixed crystals thereof.
The grain growth within a melting phase is desired, for example, to improve the electrical properties of perovskite ceramics. The addition forming the melting phase with the basic starting material is substantially not incorporated in the perovskite lattice. The liquid phase is capable of dissolving certain parts of the solid perovskite phase and precipitating them again in another location.
"Substantially not incorporated in the pervoskite lattice" means that in perovskite ceramic sintered at sintering temperatures &lt;1200.degree. C., elements from compounds forming the melting phase are not detectable in the crystal lattice, and that in perovskite ceramic sintered at sintering temperatures &gt;1200.degree. C. only traces of these elements are detectable in a quantity of .ltoreq.0.1%. The described method of manufacturing ceramic sintered bodies could almost be considered as a crystal growth from the melt which in this case is performed on the microscale in the ceramic structure. The melt flux is present, in this method, at the grain boundaries and embeds the microcrystals of the ceramic substantially without penetrating into the crystals. This accelerated diffusion transport between the grains of the perovskite phase may be considered as the basis for the increased growth and for the activated sintering behavior.
Extensive studies have demonstrated that the recrystallization process in barium titanate is initiated by a nucleation process. Individual grains grow substantially more rapidly than most of the grains surrounding them so that these preferred grains (or nuclei) finally fill the whole microstructure due to their strong growth.
The number and distribution of these so-called nuclei determine the average grain size of the recrystallized barium titanate. It is to be noted that the exact nature of the nuclei is not clear from a scientific point of view. Some authors suspect that only grains with crystal defects become active as nuclei. Others suspect that grains which, within the given grain size distribution, exceed a given size serve as nuclei.
A disadvantage of ceramic sintered bodies generally, and not only in those having a perovskite structure, is that the grain growth occurs statistically and discontinuously. The average grain size in a desired direction, hence toward a given effect, cannot substantially be influenced.